The present invention relates to the color imaging arts. It finds particular application in calibrating color printers and refining color correction tables and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in other printing devices such as multi-function devices and plotters.
Computers and other electronic equipment have typically generated three-dimensional RGB (red, green, blue) color signals. Many printers, however, receive four-dimensional CMYK (cyan, magenta, yellow, and black) signals as input and print output colors which are measured as corresponding RGB values. A look-up table is commonly provided to convert each digital RGB color signal value to a corresponding digital CMYK value before being received by the printer.
A printer which has an ideal dye behavior has a one-to-one correspondence of cyan-to-red, magenta-to-green, and yellow-to-blue. This means that when printed, the cyan ink will only absorb red light, the magenta ink will only absorb green light, and the yellow ink will only absorb blue light. However, printers inherently have a non-ideal dye behavior and therefore have a complex non-linear calorimetric response. Interactions between the cyan, magenta, and yellow inks exist which result in unwanted absorptions of reds, greens, and blues. Even once a printer is calibrated such that one or a range of input digital CMYK values produce the proper color(s), the full spectrum of CMYK values and printed colors is not accurate. In other words, the colors asked to be printed and the actual colors printed are not the same.
This discrepancy arises because the relationship between digital values that drive the printer and the resulting calorimetric response is a complex non-linear function. Labeling the response, or other values, as "colorimetric" indicates that the response or value has been measured by an instrument. Modeling the colorimetric response to achieve linearity across the available spectrum usually requires many parameters. Typically, a color correction look-up table is built which approximates the mapping between RGB colorimetric space and CMYK values. Each RGB coordinate is typically represented by an 8-bit red value, an 8-bit green value, and an 8-bit blue value. Although the RGB coordinate is capable of addressing a look-up table having 256.sup.3 locations, measuring and storing 256.sup.3 values is expensive. The look-up table is typically partitioned into a smaller size such as 16.times.16.times.16 (4096) table locations, each of which stores a four-dimensional CMYK value. Other CMYK values are then found by interpolating the known CMYK values using an interpolation process, for example, trilinear or tetrahedral interpolation.
The look-up table is built by sending a set of CMYK digital values to the printer, measuring the colorimetric RGB values of the resulting color patches outputted by the printer, and generating the look-up table from the difference between the inputted values and the measured outputted values. More specifically, the color correction look-up table corrects for non-linearities and unwanted absorptions of inks such that the printer prints the true corresponding color.
After the color correction table is generated, the printer response tends to drift over time. To correct for the drift, the device is adjusted or recalibrated periodically. Recalibrating the correction table involves printing and remeasuring a set of test color patches which are then compared to an original set of color patches by calibration software. Remeasuring, however, is typically performed by a scanner or other measuring device which is remote from the printer being recalibrated. In this case, an operator must manually reconfigure the scanner and calibration software to properly recognize and measure the test color patches. This assumes that the operator can properly identify the test color patches being tested in accordance with the original printer and its test pattern properties. Furthermore, once a color correction table is generated, it must be associated with the correct printer, otherwise, a different printer will be recalibrated with an incorrect correction table.
The present invention provides a new and improved method of calibrating a color printer which overcomes the above-referenced problems and others.